Radiation-sensitive dye bleach systems are well known and include photosensitive systems and heat sensitive systems. Heat sensitive, dye bleach systems have found utility in thermographic imaging and for antihalation applications in light sensitive elements.
Known heat sensitive dye bleach systems suitable for thermographic imaging including thermochromic compounds disclosed in British Patent Specification No. 1 356 840 and systems comprising hexaamine-cobalt (III) complexes and a pyrylium dye are disclosed in Research Disclosure, September 1980 page 366. U.S. Pat. No. 3,852,093 discloses the use of quinoneimine dyes and a mild reducing agent, U.S. Pat. No. 3,609,360 discloses an acid release process and U.S. Pat. No. 3,684,552 discloses a base release process. All of these systems providing a route for thermo-imaging.
The use of antihalation and acutance dyes to improve the imaging sharpness in photographic systems by absorbing unwanted scattered or reflected light from the base or light sensitive layer of an element is well known. The dyes are usually removed or bleached to a colourless state during or after processing of the element.
Dry silver systems which comprise a thermally developable photosensitive mixture of light sensitive silver halide with a silver salt of an organic fatty acid, e.g. behenic acid, are known and disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075. Dry silver systems also require antihalation and/or acutance in order to ensure a sharp image, which dyes must be stable under the manufacture and storage conditions of dry silver but readily bleachable during or after the heat development step. Known dyestuffs and processes suitable for antihalation applications in dry silver systems include thermally bleachable dyes as disclosed in U.S. Pat. Nos. 3,745,009, 4,033,948, 4,088,497, 4,153,463, 4,283,487, 3,615,432 and 4,197,131; photobleachable o-nitroarylidene dyes as disclosed in U.S. Pat. No. 4,028,113; and thermochromic dyes as disclosed in U.S. Pat. No. 3,769,019.
In general, the known antihalation dyes and processes for use in dry silver systems suffer from one or more of the following disadvantages. They may have a limited scope of application and must be used in specific types of dry silver formulations, they may have a post-bleach residue which causes undesirable background colouration, they may be limited to their use in a layer separate from the light sensitive layers or must be used within the light sensitive layer, and certain of the useful dyes require a long complex synthetic route for their synthesis.
Th. Zinke, Ann., 330, 361 (1904) and Th. Zinke et al, ibid, 333, 296 (1904) disclose the preparation of crystalline, deeply coloured salts of 5-anilino-N-phenyl-2,4-pentadienylideniminium chloride and the property of the salt to undergo ring closure upon heating to yield phenylpyridinium chloride and aniline: ##STR2##
J. C. McGowan, J. Chem. Soc., 777 (1949) and K. G. Lewis and C. E. Mulquiney, Tetrahedron, 33, 463 (1977) disclose a similar ring closure reaction: ##STR3##
Cyanine dyes having structures similar to formulae (A) and (B) above are extensively reported in the patent literature and are often referred to as streptocyanines. Such dyes have been disclosed as intermediates for the synthesis of oxonol dyes in U.S. Pat. No. 3,933,798 and British Patent Specification No. 1 338 799, as sensitising dyes for photographic elements in U.S. Pat. No. 3,369,904 and as antihalation or filter dyes in silver halide photographic materials which decolourise in the developing solutions in British Patent Specification No. 632 640. U.S. Pat. No. 3,627,527 discloses the use of streptocyanine dyes as sensitising dyes for organic photoconductors and discloses that the dyes undergo an absorption shift or become substantially decolourised upon heating when employed in sensitising amounts.
However, heretofore it has not been appreciated that a certain group of streptocyanine dyes bleach sufficiently cleanly and irreversibly upon heating to allow their use as heat bleachable antihalation or acutance dyes and as the image-forming component of a thermographic system.